The present invention relates to a liquid crystal display device, and more particularly to an active matrix type liquid crystal display device using thin film transistors (TFT).
In an active matrix type liquid crystal display device, a TFT (thin film transistor) is used as a semiconductor operating element (switching element). An inverted staggered structure is generally used for the structure of the TFT, wherein gate electrodes are formed on a substrate and signal lines and other electrodes are formed thereon. Practically, scan signal lines (such as gate lines and gate electrodes) are formed on a transparent insulating substrate, a gate insulating layer is formed thereon, a semiconductor layer is formed on the upper portion of the gate insulating layer, drain electrodes (data lines) and source electrodes are formed on the semiconductor layer, and transparent pixel electrodes are connected to the source electrodes. An image signal voltage is supplied to the drain electrodes (data lines). A liquid crystal display device using such a TFT structure is disclosed, for instance, in JP-A-2-48639 (1990).
When a driving circuit is formed on a periphery of the substrate, any of two kinds of mounting methods, such as TCP (Tape Carrier Package) and COG (Chip On Glass) methods, have been used.
Conventionally, the TCP method has been used more widely than the COG method. However, currently, the COG method is becoming to be used more widely in view of its superiority in cost reduction, finer pitch formation, and other advantages.
In accordance with the COG method, there are three kinds of connections, such as a connection of the input terminal of the driving circuit chip with the substrate, a connection of the output terminal of the driving circuit chip with the substrate, and a connection of the substrate with a FPC (Flexible Printed Cable) which is connected to an external circuit. And, an ACF (Anisotropic Conductive Film) is inserted at each of the above connections so that the connection is established via the ACF. In order to ensure reliability of the connection in the above case, an indium tin oxide (ITO) film, which is composed mainly of indium oxide to which tin oxide is added, is used as a wiring material of the substrate.
The liquid crystal display device using TFT technology has a feature that the display quality is superior, because active driving is possible. However, the process for forming a TFT onto the substrate is complex, and at least six separate photolithograpy processes are required. The reason for this is that six kinds of fundamental elements (film) are required for constructing the TFT, such as a scan signal line (gate lines), a gate insulating film, a semiconductor layer, a drain electrode (data line, generally as same as source electrode), a transparent pixel electrode, and a passivation layer, and patterning of each of the above films is performed by photolithography. Furthermore, because the thickness of all of the above films is several hundreds nm, and the width of the line formed by the patterning is small, such as in the order of 10 .mu.m, breakage of the lines (particularly, the data line and transparent pixel electrode) readily occurs, when they are contaminated with foreign particles, or at a portion crossing over a step on the base, and, if the patterning is not performed precisely in the photolithography process, a short circuit between electrodes or lines is readily created. If surface contamination during the process is added as a further consideration, it is seen that the electrical contact resistance between the source electrode and the transparent pixel electrode, and the gate line, or the data line, and the transparent pixel electrode at the terminal portion around the substrate, is increased significantly. Once the above phenomena are generated, defects in the display of data by the liquid crystal display device result. That means that the production yield of the substrate is decreased, and the production cost is increased.
As one of the conventional methods of increasing the production yield, a structure has been proposed wherein an insulating film between layers (passivation layer) is provided on the source electrode and the drain electrode of the TFT, and the source electrode and the drain electrode are connected with the pixel electrode via an opening provided in the insulating film between layers. In such a structure, a short circuit between the source electrode and the drain electrode with the pixel electrode, which is readily created when these electrodes are formed on the same plane, can be prevented.
However, in the conventional liquid crystal display device, decreasing the contact resistance at the connecting portion of each of the elements, which form conductive paths for electrical signals, has not been considered thoroughly when mounting the driving circuit, the TFT, and other elements on the substrate of the liquid crystal display device, and, particularly, a problem, such as low production yield of the terminal connecting portion, has not been solved. That is, in order to decrease the contact resistance when the driving circuit chip is connected with the drain line, the drain line is composed of Cr (chromium), an ITO film is arranged at the surface of the driving circuit chip, and the drain line and the driving circuit chip are contacted all through the surface so as to increase the contact area. On the other hand, the area of the region, other than the displaying region of the display panel (peripheral area), is required to be decreased. However, if the area of the contacting portion is decreased in order to meet this requirement, the contact resistance can not be decreased, because the specific contact resistance (the contact resistance per unit area) between ITO and Cr is high. If the area of the contacting portion is increased, water is liable to enter through a defect, such as a pin-hole and the like, in the resin used for packaging, and degradation of the contacting portion may readily occur.